A strong desire for battery systems with energy densities beyond conventional Lithium ion (Li-ion) chemistries exists. Lithium-sulfur batteries are a preferential choice because of their higher specific capacity as well as the abundance and low cost of elemental sulfur. A typical Li—S battery cell consists of lithium as the negative electrode, a sulfur-carbon composite as the positive electrode and an organic liquid electrolyte. Typically, Li—S batteries offer specific capacities up to 1675 Ah Kg−1 and energy densities up to 200 Wh L−1. Specific capacity is typically the total Amp-hours (Ah) per kilogram available when the battery is discharged at a particular discharge current, and the energy density is the Watt-hours (Wh) per liter. These batteries currently deliver energy densities of 350 Wh/Kg already passing the densities of conventional Li-ion batters at 180 Wh/Kg. However, these batteries have issues with short cycle lives, low charging efficiency, high self-discharge rates, and safety concerns.
Many of these problems stem from dissolution of lithium polysulfide (PS, Li2Sn), a family of sulfur reduction intermediates, in the liquid electrolyte. In spite of the problems of dissolution, the process is necessary to properly operate a Li—S battery. During the discharge step, lithium ion transport occurs through the liquid electrolyte from the anode to the cathode and yields Li2S8 by reaction of lithium and sulfur around 2.2-2.3 Volts. Generally, both elemental sulfur and its reduction products are non-conductive, so that the conductive carbon surfaces must provide deposit sites for the reduction of sulfur and lithium polysulfides. Ideally, eventual dissolution of the lithium polysulfides re-exposes the conductive carbon surfaces.
However, the lithium polysulfide species dissolved at the cathode electrode can also diffuse through the electrolyte to the lithium anode and form insoluble lithium polysulfide species. This parasitic reaction by what is sometimes referred to as ‘PS redox shuffle’ causes the loss of active material, corrosion of the lithium anode, and a shortened cycle life. Further, fire hazards exist during the battery cycling due to the presence of metastable lithium metal in flammable organic liquid electrolytes and lithium dendrites formed from the lithium that have penetrated the separator.